U.S. Pat. No. 3,223,518 and Dutch Pat. No. 132,686 disclose a process for the preparation of a granular fertilizer having a coating of a water-insoluble resin around the fertilizer, and this coating delays the release of the fertilizer. The coating comprises several layers, each formed from a solution of a copolymer of dicyclopentadiene and either linseed oil or an alkyd resin based on a soy bean oil. Such a composition is sold in the market under the registered trademark "Osmocote", a trademark of Sierra Chemical Company.
A published Dutch patent application No. 8204992 discloses a process for the preparation of fertilizer in which a controlled start for the release of the fertilizer is provided by applying to the fertilizer core a layer of a water repellent which is nearly insoluble in water, or a water swelling agent. The fertilizers may comprise trace-elements, and the coating is preferably applied to a thickness of 10.sup.-8 m to 1 cm. With this composition it is possible to control the release of the fertilizer so that the release starts preferably after 30 days and up to about 180 days. Because of the fact that in this patent application no specific choice is made regarding the coating material to be supplied (see page 5 of Dutch patent application No. 8204992) it is acceptable for the delaying action for the release of the fertilizer to be determined by the thickness of the coating.
The fertilizer composition being sold under the registered trademark Osmocote (Sierra Chemical Company), gives a reasonable release of fertilizers such as potassium, nitrogen and phosphorus compounds. There is a comparatively rapid release during the first few days, then a decreasing rate of release during about ten days and finally a constant rate of release at a reasonably high level. It also is important that, besides the controlled release of the fertilizer, the granular fertilizer be exhausted as much as possible after a certain time, so that after that period of time, as much fertilizer as possible is removed from the granule and is supplied to the plants or to the ground.
In practice, it now appears to be desirable for the rate of release of the fertilizer to be controlled in a way that supplies more fertilizer to the plant when its growth increases. For young plants it is desired that they grow in a soil having a sufficient amount of fertilizer; so the aim is to have a substantially high release rate of fertilizer during the first days and then a decreased rate to a reasonable but still high level.